Users typically subscribe to an operator of a communication network and are then entitled to access the operator's communication network which is also called home communication network. An operator typically has agreements with other operators having own (or shared) communication networks such that access to a terminal of the subscribed user can be provided via so-called serving communication networks operated by one or more other operators. A home communication network covers typically a certain region, e.g. country, and when a terminal moves out of the coverage of the user's home communication network it has to access a serving communication network of the same operator or another operator which is also known as roaming. A serving communication network for a roaming terminal is also called a visited communication network.
Communication technologies have been growing fast in history, however, the path of the evolution has not followed a monolithic and homogeneous technology trend. For example, mobile communication networks are currently evolving from pure circuit switched (CS) networks towards Internet protocol (IP) based networks, and by that integrate into IP based infrastructures that are also used for the Internet, the World Wide Web and the datacom industry. With this trend mobile networks closely follow the evolution steps of wireline communication networks, where Voice over IP (VoIP) via Digital Subscriber Line (DSL) access or via Wireless Local Network (WLAN) access is existing technology today.
Mobile operators that install IP Multimedia Subsystem (IMS) networks and offer IMS services want to make these services available also to mobile terminals of users having access to Global System for Mobile Communications (GSM) or Wideband Code Division Multiple Access (WCDMA) communication networks. Hereto, calls from and to this group of mobile terminals of GSM/WCDMA users are routed through the IMS network in order for the IMS service engine to execute the call and call related services. This concept of enabling IMS services when using CS access networks is called IMS Centralized Services (ICS), see also 3GPP Technical Specification 23.292 herein incorporated by reference, and is complemented by IMS Service Continuity (SC), see 3GPP Technical Specification 23.237, herein incorporated by reference, which is an IMS service supporting the use of session transfer mechanisms (also denoted nowadays as “access transfer mechanisms”, e.g. see newer versions of 3GPP Technical Specification 23.237) to maintain service continuity in the event of terminal mobility and/or mobility between terminals.
IMS SC is a home communication network based IMS application which provides intra-terminal transfers of one or more components of IMS multimedia sessions across different access networks and requires a Service Centralization and Continuity (SCC) Application Server (AS) and terminals with SC capabilities.
A session as known in computer science, also known as a communication session, may be regarded as a semi-permanent interactive information exchange between communicating devices that is established at a certain time and torn down at a later time. Examples are:                TCP sessions,        Web session (HTTP sessions), using session cookies and web server session management, can be either client-side or server-side. The latter may be provided by e.g. Java Session Beans, which are web server-side components that may be either stateful or stateless,        Sessions established by some session layer protocol.        
Examples for a Session protocol are:                Session Description Protocol, a format for describing streaming media initialization parameters        Session Initiation Protocol, a computer network protocol often used for IP telephony, and for setting up and tearing down peer-to-peer communication sessions consisting of one or several media streams        
A session comprises signaling data and media data and the signaling data is routed via the SCC AS. Routing the signaling data via the SCC AS is also known as anchoring. The signaling of all sessions related to a terminal are centrally anchored at the SCC AS in the home communication network in order to enable IMS SC to transfer one or more sessions of the terminal from a first access network to a second access network, e.g. from a CS access network to a packet switched (PS) access network or vice versa.
However, terminals in a serving communication network can experience a low quality of service as all session transfer requests have to be routed from the serving communication network to the home communication network where they can then be processed accordingly by the SCC AS. Long delay times as well as complex routings may worsen the service experience. For example, a voice call interruption time may increase or a data transfer may be interrupted.